The present invention is directed to methods of treatment utilizing pharmaceutical compositions comprising epibatidine and/or synthetic derivatives thereof, wherein the utility of the composition is based upon the fact that the active compounds have been found to be cholinergic receptor agonists. ##STR1##
Epibatidine has the following structure. Epibatidine was first isolated by Daly et al. from skins of the Ecuadoran poison frog, Epipedobates tricolor (Daly, et al., J. Am. Chem Soc., 102:830 (1980)). Its structure was determined by mass spectroscopy, infrared spectroscopy, and nuclear magnetic resonance as exo-2(6-chloro-3-pyridyl)-7-azabicyclo[2.2.1]-heptane (1) (Spande, et al., J. Am. Chem. Soc., 114:3475 (1992)). This alkaloid has been shown to be a potent analgesic with a nonopioid mechanism of action. The analgesic effect of epibatidine was approximately 200-times higher than morphine using the hot plate assay, and approximately 500-fold that of morphine in eliciting the Straub-tail response. The epibatidine-induced analgesia was not blocked by the opioid receptor antagonist naloxone. Furthermore, it has been determined that epibatidine had a negligible affinity for the opioid receptor (1/8000 times that of morphine). See, Spande, et al., J. Am. Chem. Soc., 114:3475 (1992).
The invention described herein is based on the discovery that epibatidine is a natural alkaloid nicotinic cholinergic receptor agonist. Other natural alkaloids are nicotine, first isolated from leaves of tobacco in 1828, and lobeline, first isolated from Lobelia inflata (India tobacco) in 1915. See, Taylor, Goodman and Gilman's The Pharmacological Basis of Therapeutics, 18th Ed., Gilman et al., eds., Pergamon Press, pp. 166-186 (1990). Nicotine is both a ganglionic and skeletal muscle receptor agonist and has been found to exert a potent analgesia on thermal stimuli as measured by the hot-plate or tail-flick test in both rats and mice (Tripathi, et al., J. Pharmacol. Exp. Therap., 221:91 (1982); Sahley et al., Pschopharmacology, 65:279 (1979); Colley, et al., Pharmacol. Biochem. Behav., 36:413 (1990); Christensen, et al., J. Neural. Transm. GenSec., 80:189 (1990)).
Differences in responses mediated by acetylcholine result from actual differences in cholinergic receptors. Responses evoked by acetylcholine are described as being nicotinic or muscarinic, which have led to the subclassification of cholinergic receptors as nicotinic cholinergic receptors or muscarinic cholinergic receptors. The response of most autonomic effector cells in peripheral visceral organs is typically muscarinic, whereas the response in parasympathetic and sympathetic ganglia, as well as responses of skeletal muscle, is nicotinic. The nicotinic receptors of autonomic ganglia and skeletal muscle are not homogeneous because they can be blocked by different antagonists. Thus, d-tubocurarine effectively blocks nicotinic responses in skeletal muscle, whereas hexamethonium and mecamylamine are more effective in blocking nicotinic responses in autonomic ganglia, thereby confirming heterogeneity in nicotinic cholinergic receptors (named N.sub.M and N.sub.N receptor respectively).
Muscarinic receptors may also be divided into at least two subtypes, M.sub.1 and M.sub.2. In general, muscarinic cholinergic receptors with the pharmacological profile characteristic of the M.sub.1 subtype are found in autonomic ganglia and in the CNS, whereas M.sub.2 muscarinic receptor exist at neuroeffector junctions of organs innervated by the parasympathetic system.
Nicotinic receptors are ligand-gated ion channels, and their activation always causes a rapid increase in cellular permeability to Na.sup.+ and K.sup.+, depolarization, and excitation. The primary structures of nicotinic receptors from various species have been deduced by molecular cloning. (Numa et al., Cold Spring Harbor Symp. Ouant. Biol., 48: 57 (1983)). The nicotinic receptors are pentameric proteins that are composed of at least two distinct subunits. Each subunit contains multiple membrane-spanning regions, and the individual subunits surround an internal channel. One of the subunits (designated .alpha.) is present in at least two copies and forms the ligand binding site on the receptor.
Nicotinic receptors (N.sub.N) in the CNS also exist as pentamers; they are composed of only two subunits, .alpha. and .beta.. Further complexity arises because multiple forms of .alpha. and .beta. have been detected (Steinbach and Ifune, Trends Neurosci., 12: 3 (1989)). In general, each of the .alpha. and .beta. subunits is found in discrete regions of the brain.
Drugs that stimulate cholinergic receptor sites on autonomic ganglia can be grouped into two major categories. The first group consists of drugs with nicotinic specificity, including nicotine itself. Their excitatory effects on ganglia are rapid in onset, are blocked by nondepolarizing ganglionic blocking agents, and mimic the initial excitatory postsynaptic potential (EPSP). The second group is composed of agents such as muscarine and methacholine. Their excitatory effects on ganglia are delayed in onset, blocked by atropine-like drugs, and mimic the slow EPSP.
Ganglionic blocking agents impair transmission by actions at the nicotinic receptors and also may be classified into two groups. The first group includes those drugs that initially stimulate the ganglia by an ACh-like action then block them because of a persistent depolarization (e.g., nicotine); prolonged application of nicotine results in desensitization of the cholinergic receptor site and continued blockade (Volle, in: Pharmacology of Ganglionic Transmission, Kharkevich, D.A., ed. Springer-Verlag, Berlin, pp. 281-312, 1980). The blockage of autonomic ganglia by the second group of blocking drugs, of which mexamethonium and trimethaphan can be regarded as prototypes, does not involve prior ganglionic stimulation or changes in the ganglionic potentials. These agents impair transmission either by competing with ACh for ganglionic cholinergic receptor sites or by blocking the channel when it is open; therefore, the initial EPSP is blocked and ganglionic transmission is inhibited.
Parkinsonism is a clinical syndrome comprised of four cardinal features: bradykinesia, muscular rigidity, resting tremor, and abnormalities of posture and gait. Despite advances in the understanding of the pathophysiology and the treatment of parkinsonism, its cause remains unknown. Classical investigations performed in the 1950's and 1960's clearly established the basal ganglia of the brain and specifically the nigrostriatal dopaminergic system as the site of the fundamental lesion in Parkinson's disease. Abundant evidence suggests that parkinsonism is a syndrome of deficiency in the dopaminergic innervation of the basal ganglia owing to degeneration of neurons in the substantia nigra (Ehringer and Hornykiewicz et al., Klin. Wochenschr., 38: 1236 (1960)). Since dopamine does not cross the blood-brain barrier when administered systemically, it has no therapeutic effects in parkinsonism. However, levodopa, the immediate metabolic precursor of dopamine, is transported into the brain and permeates into striatal tissue, where it is decarboxylated to dopamine. Clinical studies demonstrated the value of replenishment of depleted stores of dopamine in parkinsonism.
Among the panoply of other neurotransmitters contained in the basal ganglia, acetylcholine is currently known to be of significance in the pharmacotherapy of parkinsonism. A simplistic, but useful, neurochemical model of the function of the basal ganglia suggests that the neostriatum (caudate nucleus and putamen) normally contains balanced inhibitory dopaminergic and excitatory cholinergic components (Duvoisin. Arch. Neurol., 17: 124 (1967)). Although cholinergic neurons are not damaged in Parkinson's disease, the decrease in dopaminergic activity results in a relative excess of cholinergic influence. Consequently, a second strategy for the treatment of parkinsonism is to block cholinergic activity in an attempt to restore the balance of dopaminergic and cholinergic tone in the striatum. Furthermore, dopaminergic agonists and cholinergic (muscarinic) antagonists are often combined effectively.
Many epidemiology reports have found that smokers are less likely to develop Parkinson's disease than non-smokers. Evidence supporting a possible protective role for nicotine include the findings of Janson et al., Acta Physisologica Scandinavica, 132: 589 (1988) that pretreatment with nicotine will prevent some of the damage to the extra-pyramidal system by the illicit drug MPTP which produces a Parkinson-like syndrome in human.
Another movement disorder, Tourette's syndrome, seems to be responsive to nicotine (Devor and Isenberg, Lancet, 2: 1046 (1989)). Sanberg et al., Biomedicine and Pharmacotherapy, 43: 19 (1989) and Moss et al., Life Sciences, 44: 1521 (1989) found that nicotine potentiated the therapeutic effects of haloperidol in patients with Tourette's syndrome. They also found that nicotine would potentiate haloperidol-induced hypokinesia in rats.
The mechanism of action of nicotine in movement disorder is unknown. Development of tolerance to nicotine was found in humans. It was reported that tachyphylaxis developed to nicotine-induced antinociception in rats (1.25 mg/kg, s.c.) within 10 minutes lasted for up to 14 hours; but tachyphylaxis did not develop to nicotine-induced antinociception in mice (3 mg/kg, s.c.). (Tripathi, et al., J. Pharmacol. Exp. Ther., 22: 91 (1982)). Since the antinociception of nicotine is mediated through central nicotinic receptors, the mechanism of nicotine-induced desensitization of ganglionic nicotinic receptors may explain the development of tachyphylaxis to central nicotine. Nicotine initially stimulates the ganglia by an ACh-like action, as indicated by a transient tremor, then blocks them because of a persistent depolarization (Volle, in: Pharmacolocy of Ganglionic Transmission, Kharkevich, D. A., ed., Springer-Verlag, Berlin, pp. 281-312, 1980). Furthermore, one can apply the same mechanism to elucidate the therapeutic effects of nicotine in movement disorders. Smoking or exposure to nicotine induces a persistent depolarization of cholinergic neurons in striatum, which markedly lessens or induces the loss of the response to the ACh transmitter, leading to a blockage of cholinergic activity. In addition, a large number of observations indicate that nicotine can enhance dopamine release via nicotinic cholinergic receptors located on the dopaminergic nerve is terminals. This change is correlated with an increase in the fluorescence intensity of dopamine within the zona compacta of the substantia nigra (Lichtensteiger, et al., Brain Res., 117, 85, (1976)). Nicotine, continuously administered via subcutaneously implanted minipumps, can exert protective effects on the nigrostriatal dopaminergic neurons as an increased number of dopaminergic nerve cell bodies seemed to survive. It has been hypothesized that these protective effects of nicotine are due to a desensitization of the nicotinic cholinergic receptors on the dopamine neurons, leading to a reduced firing rate, improved ionic homeostasis and thus to reduced energy demands (Janson, et al., Act. Physiol. Scand., 132: 589 (1988); Reavill, in Nicotine Psychopharmacoloy, Wonnacott, et al., eds., Oxford University Press, pp. 307 (1990)). A putative anti-Parkinsonian action of nicotine and smoking may be due at least in part to a release action of nicotine on dopaminergic nerve terminals.
The therapeutic effects of nicotine in Parkinson's disease were found more than half century ago (Moll, Brit. Med. J., 1: 1079 (1926)). Besides parkinsonism, nicotine was employed as a potential drug in the treatment of Tourette's Syndrome (another movement disease) (McConville et al., Am. J. Psychiatry, 148: 739 (1991)), ulcerative colitis (Jick et al., N. Engl. J. Med., 308: 261 (1983); Tobin et al., Gastroenterology, 93: 316 (1987), Lashner et al., Digest. Dis. Sci., 35: 827 (1990), aphthous ulcers (Bittoun, Med. J. Australia, 164: 471 (1991)), smoking cessation (Glassman and Covey, Drugs, 40: 1 (1990); Gourlay and McNeil, Med, J. Australia, 153: 699 (1990)), and body weight loss/gain (Grunberg et al., Psychopharmocology, 83: 93 (1984)). The therapeutic effects of nicotine were reviewed by Jarvik (Brit. J. Addict., 86: 571 (1991)). Agonists and antagonists of nicotine useful as smoking deterrents are reported in U.S. Pat. No. 4,966,916 (Abood, 1990). Nicotine has not generally been used as a clinical drug, particularly due to its toxicity and its low potency in the treatment of disease states including parkinsonism and other movement disorders.
Development of drugs that provide a more selective, more potent or more persistent depolarization of cholinergic neurons in the CNS than nicotine will provide a new method for the treatment of Parkinson's disease and other movement disorders.
Therefore, it is an object of this invention to provide new compounds that are cholinergic receptor ligands.
It is still another object of the present invention to provide compounds which are agonists and antagonists of muscarinic and nicotinic receptors.
It is still another object of the present invention to provide new methods for the treatment of pain.
It is another object of the present invention to provide compositions and methods for the treatment of cognitive, neurological, and mental disorders, as well as other disorders characterized by decreased or increased cholinergic function.
It is yet another object of the present invention to provide pharmaceutical compositions and new methods of treatment which of certain disease states or conditions, including movement disorders such as Parkinson's disease, Tourette's syndrome, and the like, Alzheimer's disease, ulcerative colitis and aphthous ulcer, and for other medical uses, including smoking cessation and body weight loss.